The measurement from outside of ring to outside of ring is 4 inches, from inside to inside 2 1/2 inches, you can probably get another 1/4 inch apart if you let it hang off a 1/4 inch, the rings are 3/4 inch wide, you can remove one of the rings and use it with one ring this is how I use them on my red dots
I have these on a few of my scopes and red dots, holds zero when I take them off and on, LOVE THEM, on my Osprey scope from outside to outside of rings it is 4 inches, you can adjust this for less and you can remove one of the rings for single ring mount.